The present invention relates to a data communication network system, and in particular to reconfiguration control of a network which, upon occurrence of a failure or abnormality in communication function of the network, rapidly disconnects the failure, portion to recover the communication function.
As for the communication network system comprising a plurality of stations connected via at least one ring-shaped transmission line or path, a method to counter a failure caused in a dual ring network comprising first and second ring transmission lines is described in JP-W-61-500700 entitled "Data Transmission System and Method" by Nicholas C. L. Beale et al. (International patent application No. PCT/GB84/00366), for example.
In accordance with the above described prior art, each station sends a notice frame BCN (beacon) toward downstream of the first ring when the station has detected in the first ring abnormality such as a break of the transmission path, break down of a device or simply the receipt of unrecognisable data, for example. Each station which has received BCN relays the BCN to downstream stations and sends a failure acknowledge frame ACK to the second ring. If the station which has sent BCN can not receive BCN from upstream for a predetermined time period, the station forms a loop-back transmission path (WRAPB) from the second ring to the first ring. A station which can not receive ACK in a predetermined time period since the reception of BCN forms a loop-back transmission path (WRAPA) from the first ring to the second ring.
In accordance with the above described prior art, however, a loop-back transmission path is formed under the condition that BCN or ACK is not received for a predetermined time period or longer. Therefore, it takes time longer than the above described predetermined time period to complete the countermeasure against the failure, resulting in a problem of delayed resumption of communication.
In U.S. patent application Ser. No. 826,255 filed in Feb. 5, 1986, the present inventors proposed a network reconfiguration control method, whereby a first station which has detected abnormality in the first ring and sent BCN to downstream of the first ring waits for the reception of BCN from the first ring while stopping the signal relay operation in the second ring. In accordance with this method, the first station which is sending BCN intentionally causes a failure at such a location in the second ring as to form a counterpart to the location of the true failure in the first ring. Accordingly, a second station adjoining the first station via the true failure location is able to detect abnormality in the second ring. The second station receives the BCN sent by the first station via the first ring and senses the abnormality of the second ring. As a result, the second station is able to recognize that the second station itself is the station which should form the loop-back transmission path (WRAPA) extending from the first ring to the second ring.
In the above described prior patent application, the formation of the loop-back transmission path (WRAPB) extending from the second ring to the first ring by the first station is conditioned upon continuation of the BCN waiting state in the first ring for the predetermined time period or longer.